Data transmission and data interrogation between a sending-and-receiving station which may be stationary, and individual addressable transponders, which may be provided on vehicles or the like, can be used for a large number of applications and purposes. For example, it can be used for automatic toll collection or traffic control in systems in which, for example, vehicles are automatically debited for travel across a particular zone when a transponder on the vehicle passes a sending-and-receiving station at a toll collection point.
The transponder serves to identify the vehicle and access control systems can respond to identity indicia in a read only memory, for example, of the transponder. Depending on protocol selected and comparison of transponder data with fixed station data, individual access can be provided only to selected vehicles or individuals. Similar systems can be used to access controlled areas of workplaces, institutions or the like. In that case door opening facilities may respond to a transponder carried by the individual. In tool-change systems on machine tools or the like, the individual tools can be provided with transponders which can supply data representing the tool type, the duration of use, the source, data relating to machining-error correction, etc.
For all of the aforedescribed processes, it is known to provide sending-and-receiving stations which can communicate with the transponder by inductive means. The transponder can be inductively supplied with energy by a high energy signal from this station.
Heretofore, this energy was stored in a capacitor or condenser and served to activate an oscillator in the transponder to send back to the sending-and-receiving station data stored in the transponder.
Digital information transmission techniques require a clock transmission and synchronization of the units in communication with one another, i.e. between the transponders and the sending-and-receiving station.
A known data transmission signal allows the energy signal and the clock signal to be combined.
It is also known, by resonance network detuning at the antenna region of a transponder, to effect load modulation of a high frequency signal which can be detected in the sending-and-receiving station.
In all of these systems, however, improvement is desirable and, for example, as is especially the case for the transponder, a minimum size of the unit is desirable. Energy supply utilizing a condenser or capacitor sufficient to provide the energy needs of a transponder, for example, requires a relatively large capacitor. The presence of an oscillator for the retransmission of data from the transponder from the sending-and-receiving station limits the degree of miniaturization of the transponder which is possible.
Load modulation utilizing detuning of a resonant network, requires a resonant network sensitive to an exact resonant frequency and thus circuitry to maintain a given frequency.
Such circuitry also requires, for example, temperature compensation and the components necessary for temperature compensation occupy additional space. The uses of such systems because of different technical and legal requirements in different areas has been limited.